A clot is a mass of coagulated blood. It's your body's way of repairing itself, stopping blood from flowing and beginning to heal a wound.
You can use the noun clot for any liquid that forms a solid lump or mass, though it's usually a medical term specifically related to blood. Some clots are useful, like the ones that form on skin that's been cut, while others — like those that occur inside blood vessels — can be harmful. Clot can also be a verb, meaning to clog or coagulate. The root is the German word Klotz, which means "lump or block."